A block of mass 4.0 kg is put on top of a block of mass M = 5.5 kg. To cause the top block to slip on the bottom one, while the bottom one is held fixed, a horizontal force of at least 18 N must be applied to the top block. The assembly of blocks is now placed on a horizontal, frictionless table. (a) Find the magnitude of the maximum horizontal force F that can be applied to the lower block so that the blocks will move together. (b) Find the magnitude of the resulting acceleration of the blocks. Once the blocks are on the frictionless table, I run into trouble. Namely, I don't know where to place friction in my free-body-diagrams for the two blocks. Friction definitely acts on the upper block, and I think that it points in the same direction as the force applied to the bottom block. It will also probably be maximum frictional force, which was given as 18N. But does that friction act on the bottom block? I want to say not, but it's a contact force, and the bottom block is after all in contact with the top block!